Performance analysis and design optimization of volumetric solar collectors

Abstract

A solar collector is the most straightforward way of solar thermal utilization. A volumetric solar collector (also called direct absorption solar collector) is more advantageous than a traditional surfaced based solar collector due to its volumetric absorption of solar energy by the immersed nanoparticles which leads to lower surface temperature and thus less heat loss to the ambient. However, a critical issue of the volumetric solar collectors using nanofluids is that particle agglomerations may occur during the operation which negatively affect the stable and efficient use of a volumetric solar collector. One way to avoid the particle agglomeration is to reduce the particle concentration as much as possible. Due to the approximately linear relation between the nanofluid absorption coefficient and the particle concentration, the reduction of absorption coefficient can correspondingly reduce the particle concentration. In this thesis, a group studies have been conducted to reduce the absorption coefficient for the volumetric solar collectors. First, a systematic optimization has been performed using the global optimization methods for a direct absorption solar collector (DASC) to obtain the optimal design with different constraints and objectives. The lowest absorption coefficient of a blended plasmonic nanofluid with a satisfying thermal performance has been explored which can obtain a satisfactory collector performance at the same time. Second, the spectral absorption coefficient has been optimized for a DASC with a gradient based optimization method. The results showed that the pattern of the optimal spectral absorption coefficient is dependent on the constraint on nanoparticle concentration. Third, a new design for a direct absorption parabolic trough solar collector (DAPTSC) was proposed by applying a highly reflective coating on the top half of the tube so that the optical length will be increased and thus the absorption coefficient can be reduced. Apart from reducing the absorption coefficient necessary for a volumetric solar collector, another way to reduce the particle concentration is to enhance the absorption efficiency of the nanoparticles. Accordingly, in this thesis, the absorption characteristics of nanoparticles with sharp edges was also studied which generated multiple absorption peaks and exhibited much higher solar-weighted absorption-coefficient compared to the smooth nanoparticles, beneficial for more efficient solar energy absorption.